The present invention relates to a test fixture for use in automatic test equipment for electronically checking a printed circuit board (PCB) and more particularly for such a fixture which is provided with means for contacting a first set of contact points on the board with a first set of test probes and for contacting additional points with additional probes in at least one additional operating mode using a vacuum actuated feedback mechanism.
Automatic electronic test equipment for checking printed circuit boards are well known. Hundreds of test points on the circuit board must be selectively tested. This is generally accomplished with a vacuum actuated test fixture employing a first plate with an array of upstanding, spring-mounted test probes. The probes are positioned to engage the test points on the board. The board is mounted on a second plate parallel to the first plate with holes arranged to pass the probes therethrough. Springs hold the plates apart until a vacuum between the plates pulls them together, whereupon the probes contact the test points.
For certain testing applications, it is desirable to have two or more testing modes in which only selected ones of the probes are in contact with their test points while other probes are spaced away from their test points. Prior art test fixtures have employed spring-mounted test probes having two different lengths. These have regulated the spacing apart of the plates by means of a pressure regulator for maintaining a preset partial vacuum great enough to force the plates close enough for only the long probes to make contact in a first level test mode and applying full vacuum to force the plates so close together that all the probes make contact in a second level test mode.
Fixtures using the vacuum regulator to control board position relative to the test probes rely upon the principle that a given vacuum will result in a given atmospheric pressure forcing the plates together against the springs in the vacuum chamber. This system controls only the vacuum, not the distance between plates directly. It has been found in practice that any change in atmospheric pressure, spring tension, and leaks at the various gaskets or through the circuit board results in fluctuations in the partial travel position at a preset pressure on the regulator, resulting in inconsistent performance and requiring individual adjustment of the regulator for each board in some cases.